As the term is generally used, a plasma is a gas that has been ionized such that it consists predominantly of positively charged ions and free electrons, together with excited atomic, molecular, and radical species. Plasmas are used for industrial purposes such as microcircuit etching, chemical vapor deposition, and other surface modification processes. Plasma is sometimes referred to as the “fourth state of matter.” When gas is in this state, electrons and ions are continuously recombining to form neutral (uncharged) gas molecules. Additional energy must be supplied to the gas to continue the process of dissociation of atoms and molecules into ion-electron pairs. To continuously maintain the gas in a plasma state, the processes of dissociation and recombination of ion electron pairs must of course be at least in equilibrium. A state of equilibrium can be achieved at high temperatures, typically above 2000K for most gases. When temperatures are increased above the equilibrium point, the process of dissociation occurs more rapidly than the process of recombination. This produces very high densities of electrons. Industrial processes typically require these high densities of electrons, but the high temperatures are generally incompatible with the materials being processed. Consequently, residence times must be extremely short. For example, with plasma spray techniques the residence time for materials is only milliseconds. Furthermore, the particle sizes of materials being processed must be relatively small (between about 20 and 50 microns) in order to be heated by the process. These time and size limitations severely restrict the volume of material melting or processing that can be accomplished with a plasma spray system.
As a result of these limitations, research has been conducted on creating plasmas that are sustained by “non-equilibrium ionization processes.” These processes are induced by such means as an externally applied electro magnetic field, a particle beam, or laser radiation. For example, low frequency corona discharges are often used for large web or film applications that employ plasma processing. Radio frequency systems are used for some other applications, but low-pressure microwave discharges have been shown to be more efficient at producing ion-electron pairs than radio frequency systems.
However, one of the problems with present systems that incorporate microwave generation of plasmas is difficulty in initiating the plasma. The reason for this is that a body of un-ionized gas is a very poor absorber of microwave energy, especially at the low pressures needed for plasma processing. Current methods for initiating the plasma typically involve temporarily or locally increasing the gas pressure to achieve initiation and that requires a large expenditure of energy. A further problem with present microwave systems is that after initiation of the plasma, some type of confinement system is required in order to maintain the hot zone over a specified distance or throughout a specified volume. Often a large magnetic field is used to focus and constrain the system but the presence of a magnetic field puts severe limitations on the ability to process many types of materials. However, if no confinement system is used, the hot zone rapidly cools and the effective energy volume is so small that many manufacturing processes or material deposition techniques are inefficient and costly.
What is needed is a simpler and more efficient means to initiate, maintain and confine a microwave plasma process.